Generally, in an obstacle detection device for a vehicle, an ultrasonic sensor is attached to a bumper of the vehicle to send an ultrasonic wave to a rear side of the vehicle or a corner side thereof. When the ultrasonic wave is reflected by an obstacle and received by the ultrasonic sensor, the obstacle is detected.
The ultrasonic sensor is provided with a cylindrical ultrasonic vibrator, which is arranged in a cylindrical hole portion of a case unit. A cylindrical cushion member is disposed between the case unit and the ultrasonic vibrator to absorb a vibration caused by the ultrasonic vibrator. In this case, a circular vibration surface is formed at a front end of the ultrasonic vibrator, which is provided with a piezoelectric device.
When the piezoelectric device is energized, the vibration surface can be excited to vibrate by the piezoelectric device due to a piezoelectricity effect. Thus, the ultrasonic wave is sent. When the ultrasonic wave having been sent is reflected and received by the piezoelectric device, the piezoelectric device will generate a voltage due to a counter-piezoelectricity effect. In this case, the directivity of the ultrasonic vibrator in a horizontal direction is generally same with that in a vertical direction.
On the other hand, in the case where the vibration surface of the ultrasonic vibrator is formed to have an ellipse shape, the directivity of the ultrasonic vibrator in the horizontal direction can become different from that in the vertical direction so that the performance of the ultrasonic vibrator is improved. Considering that it is preferable for the vibration surface of the ultrasonic vibrator to have a circular shape, the ultrasonic vibrator (which directivity in horizontal direction is different from that in vertical direction) having the circular vibration surface is presented, for example, referring to JP-2002-238095A.
In this case, the outer surface of the cylindrical ultrasonic vibrator is partially flattened, so that the horizontal direction and the vertical direction of the ultrasonic vibrator can be properly discerned when being mounted.
When the cushion member (in which ultrasonic vibrator is inserted) is inserted into the hole portion of the case unit, protrusion portions which are arranged at an outer surface of the cushion member will be engaged with concave portions which are formed at a surface of the hole portion of the case unit. Thus, the cushion member is circumferentially positioned and restricted from rotating.
However, in this case, the cushion member is constructed to tightly contact the hole portion of the case unit. When the cushion member is inserted into the hole portion of the case unit, the protrusion portions which are to be inserted in the concave portions are not provided with a clearance to readily enter the hole portion. Therefore, the operation performance in an insertion of the cushion member into the hole portion is impaired. That is, the insertion operation of the cushion member is troublesome.